1. Field of the Invention
The present invention is related to semiconductor materials, methods, and devices, and more particularly, to a method for enhancing growth of semi-polar (Al,In,Ga,B)N via metalorganic chemical vapor deposition.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [Ref. x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, in which the structure is described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Group III and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the würtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on non-polar planes of the crystal. Such planes contain equal numbers of Ga and N atoms and are charge-neutral. Furthermore, subsequent non-polar layers are crystallographically equivalent to one another so the crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent non-polar planes in GaN are the {11 20} family, known collectively as a-planes, and the {1 100} family, known collectively as m-planes. Unfortunately, despite advances made by researchers at the University of California, the assignee of the present invention, growth of non-polar nitrides remains challenging and has not yet been widely adopted in the III-nitride industry.
Another approach to reducing or possibly eliminating the polarization effects in GaN optoelectronic devices is to grow the devices on semi-polar planes of the crystal. The term semi-polar planes can be used to refer to a wide variety of planes that possess two nonzero h, i, or k Miller indices, and a nonzero l Miller index. Some commonly observed examples of semi-polar planes in c-plane GaN heteroepitaxy include the {11 22}, {10 11}, and {10 13} planes, which are found in the facets of pits. These planes also happen to be the same planes that the inventors have grown in the form of planar films. Other examples of semi-polar planes in the würtzite crystal structure include, but are not limited to, {10 12}, {20 21}, and {10 14}. The nitride crystal's polarization vector lies neither within such planes or normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the {10 11} and {10 13} planes are at 62.98° and 32.06° to the c-plane, respectively.
In addition to spontaneous polarization, the second form of polarization present in nitrides is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al, In, Ga, B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer latticed matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
The advantage of using semi-polar planes over c-plane nitrides is that the total polarization will be reduced. There may even be zero polarization for specific alloy compositions on specific planes. Such scenarios will be discussed in detail in future scientific papers. The important point is that the polarization will be reduced compared to that of c-plane nitride structures.
Bulk crystals of GaN are not readily available, so it is not possible to simply cut a crystal to present a surface for subsequent device regrowth. Commonly, GaN films are initially grown heteroepitaxially, i.e., on foreign substrates that provide a reasonable lattice match to GaN.
Semi-polar GaN planes have been demonstrated on the sidewalls of patterned c-plane oriented stripes. Nishizuka et al. [Ref. 1] have grown {11 22} InGaN quantum wells by this technique. They have also demonstrated that the internal quantum efficiency of the semi-polar plane {11 22} is higher than that of the c-plane, which results from the reduced polarization.
However, this method of producing semi-polar planes is drastically different than that of the current invention; it is an artifact of the epitaxial lateral overgrowth (ELO) technique. ELO is a cumbersome processing and growth method used to reduce defects in GaN and other semiconductors. It involves patterning stripes of a mask material, often SiO2 for GaN. The GaN is then grown from open windows between the mask and then grown over the mask. To form a continuous film, the GaN is then coalesced by lateral growth. The facets of these stripes can be controlled by the growth parameters. If the growth is stopped before the stripes coalesce, then a small area of semi-polar plane can be exposed. This area may be 10 μm wide at best. The semi-polar plane will be not parallel to the substrate surface. This available surface area is too small to process it into a semi-polar LED. Furthermore, forming device structures on inclined facets is significantly more difficult than forming those structures on normal planes. Also, not all nitride compositions are compatible with ELO processes; as such, only ELO of GaN is widely practiced.
Nucleation, buffer, and/or wetting layers have been extensively used in the growth of high quality nitrides since the early 1990s [Refs. 2, 3]. This technique typically employs the use of a thin layer (50 Å-2000 Å) of polycrystalline and/or amorphous nitride semiconductor material prior to the deposition of thicker (1 μm-5 μm) nitride semiconductor material. While the advantages of using nucleation layers (NLs) in heteroepitaxy of c-plane GaN thin films is well established, the mechanisms for how the NLs improve crystal quality are not well understood. It is believed that NLs provide crystal sites onto which high quality nitride materials then deposit [Refs. 4, 5]. The later deposition shows a dramatic improvement in crystal, electrical, and optical properties compared to nitrides deposited without a NL.
Although the use of NLs has been extensively documented for nitride thin films, they comprise of nitrides grown only in the (0001) or c-plane crystallographic direction [Refs. 6, 7]. Ramdani et al. [Ref. 7] demonstrated the use of a plurality of buffer layers in order to improve the crystal quality of c-plane GaN grown on a spinel substrate. This method is considerably different from the present invention in that the author is describing the growth of c-plane GaN, which has a 9% lattice mismatch to (111) spinel, as described in [Ref. 7]. It is also very cumbersome due to the plurality of buffer layers, four in total, needed to produce device quality c-plane GaN. In contrast, the current invention describes the use of a single buffer layer for the improvement of semi-polar GaN.
Improvement of c-plane GaN films has also been demonstrated by Akasaki et al. [Ref. 6]. As discussed earlier, optoelectronic and electronic devices in this particular crystallographic direction suffer from the undesirable QCSE, due to the existence of strong piezoelectric and spontaneous polarizations. The present invention distinguishes itself from the above-mentioned methods by the use of a single buffer layer in order to improve the quality of semi-polar nitride thin films.
There is a need, then for improved methods for the growth of planar films of semi-polar nitrides, in which a large area of (Al,In,Ga,B)N is parallel to the substrate surface. The present invention satisfies this need.